AJust wondering if anyone knows why smbc start to turn yellow after several hours? The frustrating part is it's not very consistent as it can look a bit mottled at times. The only way I can see to prevent this is to used organic butter. Does American buttercream yellow as well?

If you are using a hot blade to smooth your BC or smoothing it after it has been chilled, that can break the emulsion and cause yellowing. 

AThanks! That might be one reason however I had some smbc in my kitchen aid bowl which was off white but the next day it turned yellowish? The next batch I made *didnt* turn yellow even though I made it the exact same way. So frustrating, lol!

AWhen exposed to the air, butter will often turn yellow on it's outer surface - I'm not sure if this is oxidation or another process at work. Maybe you could try covering the surface of the buttercream with cling wrap like you would do a ganache or custard to prevent a skin forming - this night keep it more airtight.

AThanks for the suggestion! I had to make 15 batches of smbc and I thought the same thing you did,..so I covered the remaining batches with Saran Wrap but I still had some turn yellow? Darn!